Astronomers Find Enormous Hole in the Universe

Astronomers have found an enormous hole in the Universe, nearly
a billion light-years across, empty of both normal matter such as
stars, galaxies, and gas, and the mysterious, unseen "dark matter."
While earlier studies have shown holes, or voids, in the large-scale
structure of the Universe, this new discovery dwarfs them all.

Hole in Universe revealed by its effect
on Cosmic Microwave Background radiation.CREDIT:Bill Saxton, NRAO/AUI/NSF, NASA
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graphics
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"Not only has no one ever found a void this big, but we never even
expected to find one this size," said Lawrence Rudnick of the
University of Minnesota. Rudnick, along with Shea Brown and Liliya
R. Williams, also of the University of Minnesota, reported their
findings in a
paper accepted for publication in the Astrophysical
Journal.

Astronomers have known for years that, on large scales, the
Universe has voids largely empty of matter. However, most of
these voids are much smaller than the one found by Rudnick and
his colleagues. In addition, the number of discovered voids
decreases as the size increases.

"What we've found is not normal, based on either observational
studies or on computer simulations of the large-scale
evolution of the Universe," Williams said.

"We already knew there was something different about this spot
in the sky," Rudnick said. The region had been dubbed the "WMAP
Cold Spot," because it stood out in a map of the
Cosmic Microwave
Background (CMB) radiation made by the
Wilkinson Microwave Anisotopy
Probe (WMAP) satellite, launched by NASA in 2001. The CMB, faint
radio waves that are the remnant radiation from the Big Bang, is the
earliest "baby picture" available of the Universe. Irregularities
in the CMB show structures that existed only a few hundred thousand
years after the Big Bang.

The WMAP satellite measured temperature differences in the CMB
that are only millionths of a degree. The cold region in Eridanus
was discovered in 2004.

Astronomers wondered if the cold spot was intrinsic to the CMB,
and thus indicated some structure in the very early Universe,
or whether it could be caused by something more nearby through
which the CMB had to pass on its way to Earth. Finding the dearth
of galaxies in that region by studying NVSS data resolved that
question.

"Although our surprising results need independent confirmation, the
slightly colder temperature of the CMB in this region appears to be
caused by a huge hole devoid of nearly all matter roughly 6-10 billion
light-years from Earth," Rudnick said.

How does a lack of matter cause a cooler temperature in the
Big Bang's remnant radiation as seen from Earth?

Photons of the CMB gain a small amount of energy when they
pass through a region of space populated by matter. This effect
is caused by the enigmatic "dark energy" that is accelerating
the expansion of the Universe. This gain in photon energy makes
the CMB appear slightly warmer in that direction. When the photons
pass through an empty void, they lose a small amount of energy from
this effect, and so the CMB radiation passing through such a region
appears cooler.

The acceleration of the Universe's expansion, and thus dark
energy, were discovered less than a decade ago. The physical
properties of dark energy are unknown, though it is by far the
most abundant form of energy in the Universe today. Learning its
nature is one of the most fundamental current problems in
astrophysics.

The NVSS imaged the roughly 82 percent of the sky visible from the
New Mexico site of the VLA. The survey consists of 217,446 individual
observations that consumed 2,940 hours of telescope time between 1993
and 1997. A set of 2,326 images was produced from the data, and these
images are available via the NRAO Web site. The survey also produced
a catalog of more than 1.8 million individual objects identifiable
in the images. The NVSS has been cited in more than 1,200
scientific papers.

NASA's WMAP satellite, using microwave amplifiers produced by
NRAO's Central Development Laboratory, has yielded a wealth of
new information about the age and history of the Universe, the
emergence of the first stars, and the composition of the Universe.
WMAP results have been extensively cited by scientists in a wide
variety of astrophysical specialties.